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Cloud (ST cloud of loose or fibrous)

ST. cloud of loose or fibrous structure, feathery in appearance and of great variety-said to have been seen at an altitude of 10 miles. Its particles are supposed to be frozen and crystallized, which give rise to halos, coronæ, and other optical appearances. Cirrus is said to be often the precursor of windy weather or changes. Their movement is generally different from that of the lower clouds.

Cumulo-stratus-A modification of cumulus; flat-topped, mushroom-shaped masses; have a tendency to spread and overcast the sky, and indicate coming rainy weather. Cirro-stratus-A fibrous cirrus cloud in close horizontal arrangement.

Sky mottled with these clouds is known as mackerel sky. Often precedes wind and rain. Cirro-cumulus-Small, round masses, disposed with more or less regularity; usually a high level cloud, though below that of cirrus. They appear most frequently in dry and warm weather.

Nimbus-A low cloud, from which rain is falling, the masses so blended together as to form no definite outline. Scud-A term referring to low, detached clouds, drifting rapidly before the wind.

Precipitation, or rain, is one of the most irregular of all meteorological phenomena, there being places of habitual dryness throughout the year with but occasional sprinklings. Tropical countries generally have a dry and a wet season, and there are other localities where rain may fall irregularly at all seasons. There are places where the annual rain-fall amounts to several hundred inches. Fifty inches per annum may be regarded as a moderate rain-fall, and below 20 inches a very light one, and generally insufficient for agricultural purposes unless it should fall in the right season.

The study of clouds and their movements has, in recent years, been greatly extended through international agreements and co-operation. The development of aeronautics has greatly advanced our knowledge regarding clouds and, on the other hand, has made more accurate and extensive knowledge regarding them a matter of practical necessity rather than theoretical interest.